Why Does PH3 Show Little or No Hybridization Compared to PF3?
Understanding the Hybridisation of PH3 (Phosphine) is crucial for mastering chemical bonding in JEE Main Chemistry. Although PH3 is theoretically assigned sp3 hybridization by the steric number method, its actual bonding and geometry present an important exception students must grasp for exams. The structure and bond angles of PH3 reveal why hybridization, as usually applied, fails in this molecule, setting it apart from classic cases like NH3 and PF3.
The basic idea of hybridization is the mixing of atomic orbitals (s and p) in the central atom to form a new set of equivalent hybrid orbitals for bonding. In theory, phosphorus in PH3 (valence shell: 3s2 3p3) would combine its 3s and three 3p orbitals to give four sp3 hybrids, just like in ammonia (NH3). Each hydrogen atom’s 1s orbital would then overlap with one PH3 hybrid orbital, leaving one orbital for the lone pair—implying a tetrahedral (or trigonal pyramidal) geometry with expected bond angles close to 109.5°.
However, real-world measurements and deeper theory show that PH3 does not fully use sp3 hybridisation. Instead, its bonding involves mostly unhybridized 3p orbitals, with the lone pair residing in the 3s orbital. This leads to a much smaller observed bond angle (~93.5°) and significant deviation from ammonia—even though both are group 15 hydrides. The low bond angle, unhybridized p-orbital bonding, and lone pair behavior are byproducts of phosphorus’s lower electronegativity (2.19), larger size, and Drago’s rule conditions.
PH3 Hybridization: Theory vs. Reality
Drago’s rule provides a practical guide for predicting when sp3 hybridization is ineffective in heavier p-block hydrides like PH3:
- Central atom is from period ≥3 with low electronegativity (≤2.5).
- One or more lone pairs are present.
- Size of central atom causes poor s-p orbital mixing.
- Sigma bonds form primarily using pure p-orbitals.
Applying these criteria, PH3 stands as a classic Drago molecule. The phosphorus atom in PH3 has one lone pair (3s) and forms three P-H bonds using mainly 3p orbitals without significant hybridization.
| Molecule | Predicted Hybridization | Actual Bonding/Overlap | Bond Angle (°) |
|---|---|---|---|
| PH3 | sp3 (by steric number) | Mainly 3p (P) + 1s (H); lone pair in 3s | 93.5 |
| NH3 | sp3 | 3s/2p well-mixed; hybrid orbitals | 107.5 |
| PF3 | sp3 | Significant hybridization (high P–F overlap) | 97 |
This table highlights how PH3 diverges from both theoretical predictions and structurally similar molecules when considering effective orbital overlaps and angles. The bond angle sequence (NH3 > PF3 > PH3) is a common exam application.
Structural Details and Bonding in PH3
The molecular geometry of PH3 is trigonal pyramidal, matching its electron domain count (three bonded pairs + one lone pair). But unlike methane or ammonia, the P-H bonds in PH3 form by lateral overlap between the phosphorus 3p orbitals and the hydrogen 1s orbitals. The lone pair occupies a nearly pure 3s orbital, staying more localized and less available for bonding.
- The bond angle in PH3 is about 93.5°, close to a right angle due to poor s–p mixing and limited lone-pair–bond-pair repulsion.
- Lone pair is almost fully non-bonding, explaining PH3’s low basicity compared to NH3.
- Actual hybridization is best described as “nearly none” or pure p-orbital bonding—an exception to shortcut formulas.
Remember, using the formula (steric number = number of sigma bonds + lone pairs) gives SN = 4 for phosphorus in PH3. However, Drago’s rule overrides this for heavier elements, a classic point for tricky JEE questions.
Other group 15 hydrides (NH3, AsH3, SbH3, BiH3) show a similar down-the-group drop in bond angle, matching the decrease in effective hybridization due to poor orbital energy and size compatibility. Contrast this with PF3 hybridization, where fluorine’s high electronegativity pulls the phosphorus electrons, enabling stronger s–p mixing and more ideal tetrahedral angles.
PH3 vs PF3 vs NH3: Exam Comparison
- NH3: True sp3 hybridization; strong N–H overlap; large bond angle (107.5°); basic.
- PF3: sp3 hybridized due to high F electronegativity; bond angle ~97°.
- PH3: Minimal hybridization; mainly p-orbital bonds; small bond angle (93.5°); weakly basic, lone pair stays in s orbital.
Examiners often exploit these subtle distinctions in hybridization for MCQs. Watch for molecular formula tricks and “which statement is false/true” wording.
Hybridisation of PH3 in the JEE Syllabus
Understanding hybridization in phosphine links several JEE chemistry concepts—molecular structure, VSEPR theory exceptions, group trends, and typical traps in shortcut formulas. Remember:
- For PH3, do not accept “sp3” hybridization at face value. Justify using Drago’s rule and bond angle data.
- The lone pair in PH3 is not easily available for donation (coordination), unlike NH3.
- Experimental evidence (IR, NMR, electron diffraction) confirms the theoretical model with ~93° angle.
For revision, always check whether the central atom belongs to the third period or higher and has at least one lone pair: these often show little or no hybridization. This makes PH3 a favorite for conceptual MCQs and last-minute flashcards. For more on bridge concepts, see p-block elements and valence bond theory references.
Key Takeaways and Common Pitfalls
- PH3 has a trigonal pyramidal shape with ~93.5° bond angle; lone pair almost pure s orbital.
- No significant hybridization—bonding uses p-orbitals on phosphorus; Drago’s rule applies.
- Don’t use simple steric number shortcuts for heavier period 3 or 4 p-block hydrides; check for Drago’s criteria.
- Compare PH3 with NH3 and PF3 to reinforce exceptions—angles and basicity differ sharply.
- Be alert: PH3 may be called “sp3 hybridized” in old textbooks but Drago’s rule and modern evidence prevail (minimal effective hybridization).
- Revise hybridization basics and VSEPR theory to secure marks for molecular geometry questions.
- PH3’s low bond angle and weak basicity are consequence of its unique bonding; expect these facts in assertion–reason and comparison MCQs.
By mastering the Hybridisation of PH3 (Phosphine), JEE aspirants can quickly spot exceptions, avoid formula traps, and boost their confidence for chemical bonding questions. Always double-check the context for hybridization—hydrides of heavier p-block elements often break the rules. For stepwise clarity across concepts like shapes, lone pair effects, and comparison with related molecules, explore more Vedantu topic pages in chemical bonding and p-block chemistry.
FAQs on Hybridization of PH3: Concept, Structure, and Exam Insights
1. What is the hybridization of PH3?
Phosphine (PH3) exhibits nominal sp3 hybridization, but its bonds are almost pure p in character. This is due to the poor overlap between the 3s/3p orbitals of phosphorus and the 1s orbital of hydrogen, resulting in minimal hybridization. Key points include:
- Expected hybridization: sp3 (based on electron domains and VSEPR theory)
- Actual bonding: Almost unhybridized p-orbitals form bonds
- Lone pair: On phosphorus, mostly in the s-orbital
- Geometry: Trigonal pyramidal shape
2. Why does PH3 have no hybridisation but PF3 does?
PH3 shows almost no hybridization, while PF3 is sp3 hybridized due to differences in bond character and orbital overlap.
- In PH3, poor overlap between 3p (P) and 1s (H) means little need for hybridization—most bonds use pure p orbitals.
- In PF3, strong overlap between 3p (P) and 2p (F) favors effective sp3 hybridization.
- Thus, PF3 has a regular sp3 geometry, while PH3’s bonds remain largely unhybridized.
3. What is the structure of PH3?
The molecular structure of PH3 is trigonal pyramidal, with phosphorus at the apex and three hydrogens at the base.
- Lone pair: One, on phosphorus
- Bond pairs: Three P–H bonds
- Geometry: Trigonal pyramidal as predicted by VSEPR theory
- Bond angle: About 94°, much less than the 107° in NH3
4. Why is the bond angle of PH3 less than that of NH3?
The bond angle in PH3 (~94°) is smaller than in NH3 (~107°) because phosphorus’s lone pair occupies more s-character and P–H bonds are almost pure p-orbital.
- Weaker P–H bonding leads to less repulsion between bond pairs
- Lone pair-bond pair repulsion is less pronounced than in NH3
- Larger phosphorus atom size decreases the bond angle further
5. Is PF3 sp3 hybridized?
Yes, PF3 is considered sp3 hybridized with a trigonal pyramidal geometry and a bond angle close to 97°.
- All four electron groups (three P–F bonds and one lone pair) use hybrid orbitals
- Stronger overlap with fluorine atoms leads to effective sp3 hybridization
- Similar shape to PH3, but orbital character differs
6. How can I deduce PH3 hybridization for exams?
To deduce hybridization of PH3, use the formula: hybridization = (number of bond pairs + number of lone pairs on central atom).
- Phosphorus has 5 valence electrons
- Each hydrogen adds 1 electron × 3 = 3
- Total = 8 electrons or 4 pairs (3 bonds + 1 lone pair)
- Predicted hybridization: sp3, but real bonds are nearly pure p
7. Why is PH3’s bond angle smaller than NH3 even though both are nominally sp3 hybridized?
Despite both PH3 and NH3 being assigned sp3 hybridization, PH3’s bond angle is smaller due to minimal hybridization and larger atom size.
- PH3 bonds: Almost pure p orbital, less s-character, leading to smaller bond angle (~94°)
- NH3 bonds: True sp3 hybridization, higher s-character in the lone pair, bond angle ~107°
- Size effect: Larger phosphorus atom reduces electron pair repulsion
8. What error do students make when applying the hybridization formula to PH3?
The common mistake is assuming PH3 is perfectly sp3 hybridized like NH3, ignoring differences in bond character.
- Formula method predicts sp3, but does not account for poor 3p–1s overlap in PH3
- Exam tip: Always relate theory to actual structure and bond data
9. What is the difference in hybridization between PH3 and PF3?
PH3 has almost no hybridization, while PF3 is truly sp3 hybridized due to better orbital overlap with fluorine.
- PF3: Favorable 3p–2p overlap, sp3 hybridization, regular geometry
- PH3: Poor 3p–1s overlap, almost pure p bonds, much lower bond angle
- Exam point: Always check overlap quality when comparing group 15 compounds
10. Can PH3 act as a ligand due to its lone pair?
Yes, PH3 can act as a ligand in coordination compounds by donating its non-bonding electron pair from phosphorus.
- Lone pair on phosphorus can coordinate to metal centers
- PH3 forms complexes such as Ni(PH3)4
